The present disclosure relates to methods for tuning the stiffness of a structure, and in particular to methods for tuning the stiffness of a structure by changing the shape of the structure, including, for example, changing the shape of the structure by piezoelectric means. The disclosure further relates to methods for resonance frequency tuning of resonating structures, such as for example, vibration energy scavengers. Furthermore it relates to tunable devices, such as vibration energy scavengers or sensors.
Future wireless sensor networks may comprise sensor nodes which occupy a volume of typically a few cm3. The scaling down of batteries for powering such sensor nodes faces technological restrictions as well as a loss in storage density. For this reason, it would be desirable to replace batteries with more efficient, miniaturized power sources. Energy scavengers based on the recuperation of wasted ambient energy are one possible alternative to batteries. Several scavenger concepts have been proposed, including concepts based on the conversion of thermal energy, pressure energy, or kinetic energy.
Kinetic energy scavengers or vibration energy scavengers convert energy in the form of mechanical movement (e.g. in the form of vibrations or random displacements) into electrical energy. For conversion of kinetic energy into electrical energy, different conversion mechanisms may be employed, such as, for example, conversion mechanisms based on piezoelectric, electrostatic, or electromagnetic mechanisms. Piezoelectric scavengers employ active materials that generate a charge when mechanically stressed. Electrostatic scavengers utilize the relative movement between electrically isolated charged capacitor plates to generate energy. Electromagnetic scavengers are based on Faraday's law of electromagnetic induction, and generate electrical energy from the relative motion between a magnetic flux gradient and a conductor.
A vibration energy scavenger has the maximum power output when input vibrations closely match its resonance frequency. The resonance frequency of a vibration energy scavenger is determined by the material properties and the dimensions of the scavenger's parts. However, in practical applications, input vibrations may occur in a wide frequency spectrum. Therefore, the vibration energy scavenger structure stays out of resonance as the input vibration frequency changes, and it generates very low power or no power at all. It would be advantageous to have a single device that operates effectively over a range of input vibration frequencies.
Resonance frequency tuning of oscillating structures is a common practice in vibration isolation, such as in various MEMS resonators, etc. The concept is based on changing the resonance frequency of a cantilever by changing its stiffness. Several methods such as piezoelectric tuning (e.g. U.S. Pat. No. 6,336,366 and U.S. Pat. No. 6,943,484), magnetic tuning (U.S. Pat. No. 6,311,557), electrostatic tuning (U.S. Pat. No. 6,263,736) and thermal tuning (U.S. Pat. No. 4,874,215) of MEMS systems have been presented.
In U.S. Pat. No. 6,336,366, a piezoelectric tuning effect is obtained by applying a piezoelectric stack (electrode/piezoelectric layer/electrode) on a cantilever structure. When an electrical potential is applied between both electrodes of the piezoelectric stack, piezoelectric action in the piezoelectric layer changes the stiffness of the piezoelectric material, hence alters the effective stiffness of the resonating cantilever and thus changes its resonance frequency. The method is based on the stress change in the piezoelectric stack as a function of the applied electrical field and therefore the tuning effect is limited by the thickness ratio of the piezoelectric layer and the oscillating cantilever beam.
In U.S. Pat. No. 6,943,484 a tuning effect is obtained by controlling the amount of capacitive shunting achieved by an external ‘tuning capacitor’ connected in parallel to a passive piezoelectric stack on an oscillating beam. Electrically shunting the piezoelectric stack changes the effective stiffness of the beam and thus the resonance frequency. The limits of the stiffness change by capacitive shunting are at the open circuit stiffness and the short circuit stiffness, which are related by the electromechanical coupling coefficient of the piezoelectric stack. These limits are theoretically and experimentally investigated by C. Davis and G. Lesieutre in “An actively tuned solid-state vibration absorber using capacitive shunting of piezoelectric stiffness”, Journal of Sound and Vibration, 232(3), 601-617, 2000. In practice, the stiffness of the piezoelectric element is in parallel with the inherent mechanical stiffness of the whole resonator. The relative magnitudes of these two stiffnesses determine the net frequency change possible via electrical shunting. Therefore, only a fraction of the net device stiffness may be changed by electrical shunting.
In “Piezoelectric actuator integrated cantilever with tunable spring constant for atom probe”, MEMS 2006, Istanbul, 22-26 Jan. 2006, Y. Kawai et al present the use of a piezoelectric tuning actuator for changing the geometrical aspects of a cantilever structure, thereby changing the stiffness of the cantilever structure. They employed an integrated piezoelectric actuator for bending the cantilever along the lateral direction for changing its cross section profile, thus changing its stiffness. An additional piezoelectric actuator is provided for deforming the whole cantilever beam's shape in the longitudinal direction. The stiffness of the structure is changed by a change in moment of inertia. However, this method is applied on a very small structure for probing applications (e.g. AFM tips), wherein the thickness of the cantilever is in the order of a few μm (e.g. 1 to 2 μm), the length of the cantilever is typically in the order of 750 μm and the cantilever comprises 2 beams with a width in the order of 50 μm, the total width being in the order of 300 μm. The structure presented is designed for cantilever structures with a relatively low stiffness and for large actuation needs. The stiffness tuning mechanism is used for removing the artifacts of low stiffness in the scanning mode. The load at the tip of the cantilever is negligible since it is used for picking up a single atom or molecule. The relatively small dimensions and the relatively low stiffness of the structure allow bending the whole cantilever beam by piezoelectric actuators for manipulating its stiffness.
The power generated in vibration energy harvesting devices is proportional to the attached proof mass. For fulfilling the power requirements (e.g. >100 μW) and for achieving structural rigidity with a large mass, relatively large beams or cantilevers with a length and a width in the order of millimeters and a thickness in the order of tens of micrometers are needed. Therefore, the structure described by Kawai et al., wherein the cantilever structure is optimized for low stiffness applications and for large actuation needs, is not suitable for tuning the resonance frequency of vibration energy scavengers.
There are a limited number of studies related to tuning the resonance frequency of vibration energy scavengers. For example, in “Resonance tuning of piezoelectric vibration energy scavenging generators using compressive axial preload”, Smart Materials and Structures 15, 2006, 1413-1420, Leland et al. describe a method wherein axial forces are used for changing the effective stiffness of the oscillating structure. In this approach, a simply supported piezoelectric bimorph is used as an active element, with a proof mass mounted at the bimorph's center. A variable compressive axial preload is applied to the bimorph, reducing its stiffness and thus the resonance frequency of the device. The stiffness decrease is a function of the load applied in a given system. However, the change in the apparent stiffness is inversely proportional to the initial stiffness. This means that the largest tuning effect can be achieved by starting with a low stiffness. Besides the relatively low stiffness requirement, the maximum axial load applied to the structure is determined by the buckling load, which is lower in a low stiffness system. In addition, it may be difficult to fabricate the system proposed by means of micromachining methods.